FIX [`bnb`] Make `unexpected_keys` optional (#29420)
* make `unexpected_keys` optional
* push
* Apply suggestions from code review
Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>
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Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>
